1. Field of the Invention
This invention relates to digital image processing and more particularly to apparatus for rapidly performing data interpolation required for image magnification.
2. Description of the Prior Art
A digitally stored image comprises a plurality of numerical values each representing an image characteristic, typically an optical density value, for a small, discrete portion of the image. These values are known in the art as PIXEL values, the name representing a short hand form of the term "Picture Element". The relative location of each PIXEL in the complete image is also known. A captured and digitized image is stored in a memory in the form of a finite number of picture values representing image densities for each PIXEL.
In reconstructing the image for display, the stored PIXEL values are recovered and used to recreate each original picture element in the exact location in the image, with the same optical density as in the original.
So long as there is a one to one relationship between the number of acquired original picture elements and the required output picture elements there are of course no problems. When one wishes to magnify an acquired image during display, it becomes immediately apparent that since the supply of original pixel data is fixed, there will be gaps in the displayed information, which must be filled in some manner in order to produce an acceptable display.
It has been known to use the original pixel data to interpolate additional pixel values filling in the gaps in the original data generated because of the image magnification. Mathematical techniques to do the interpolation are well known.
A commonly used approach for data interpolation, relies on defining in mathematical terms a three dimensional curve using existing pixel data and from that curve calculating intermediate pixel values. Existing pixel values are obtained from a defined area surrounding the area to be filled in, and these values are used to predict additional pixel values. The number of the new pixels depends on the magnification ratio. For a magnification of 2 times, for instance, four values have to be generated for each original pixel value. The new values are calculated using a number of arithmetic operations on the available pixel data, which involve selecting a predetermined number of coefficients, multiplying a selected number of original pixel data by the selected coefficients, and adding the results to produce new pixel values.
The process of selecting the proper coefficients, obtaining the original data, multiplying and adding the result to produce new values is time consuming, and particularly so if implemented in software. A substantial speed advantage may be gained by developing hardware to do the mathematical processing. Some such hardware is disclosed in U.S. Pat. Nos. 4,468,693 and 4,837,722. While both of the above references teach circuitry that achieves rapid new pixel generation, when dealing with large image magnification ratios and particularly in situations where there is need to use multiple coefficients resulting in numerous arithmetic operations, especially when real, or quasireal, time displays are desired, increased speed is still needed.